Production of combustible gases

ABSTRACT

1,043,377. Combustible gas. GAS COUNCIL. May 27, 1965 [June 8, 1964], No. 23698/64. Heading C5E. A methane-containing gas is made by a twostage process from paraffinic hydrocarbons. The first (gasification) stage, which is a reaction with steam over a nickel catalyst, is similar to that described in Specification 1,000,309. In the second stage, the gas is catalytically steamreformed, optionally with added steam, at a temperature above 550‹ C., after being preheated in an externally fired heater, to decrease the methane content of the final gas. The second stage may comprise more than one pair of preheating and reforming steps. The catalyst used in the gasification stage may be one described in Specification 969,637. Four specific examples are described.

United States Patent US. Cl. 48-214 8 Claims ABSTRACT OF THE DISCLOSURE Low temperature steam reforming of hydrocarbon feedstocks having from 4 to 15 carbon atoms per molecule in a two-stage process consisting of gasification and reforming stages, and particularly to the second stage of this process. The gaseous mixture from the first stage is passed through at least one pair of stages consisting of, first, an externally fired preheater and, secondly, a reforming stage in which the preheated gases are subjected to the action of a reforming catalyst at a temperature above 550 C. to bring about the conversion of methane contained therein by reaction with steam to form carbon monoxide and hydrogen.

The present invention relates to processes for the production of combustible gases.

It is known to produce combustible gas containing methane, for example, town gas, and in particular town gas having a calorific value of 500 B.t.u. per cu. ft., by the gasification in steam of light hydrocarbon oils, for example, light petroleum distillate. An example of such a process is described and claimed in our British Patent No. 820,257. More particularly this specification discloses a first stage in which mixtures of predominantly paraflinic hydrocarbons containing an average of 4 to 10 carbon atoms and admixed in vapour form with steam are passed through a bed of nickel catalyst under atmospheric or superatmospheric pressure and the hydrocarbon vapour and steam are passed into the catalyst bed at a temperature above 350 C. such that the bed is maintained by the reaction at a temperature within the range 400 C. to 550 C.

The gas so produced contains more methane than is required for town gas and a second stage is disclosed in this prior specification wherein the methane-containing gas resulting from the first stage together with the nudecomposed steam are passed through a further bed of nickel catalyst operated at a temperature above 500 C. to bring about the conversion of methane by reaction with steam into carbon monoxide and hydrogen to the extent required to reduce the methane content as desired. This reaction is endothermic and the required heat can be supplied, for example, by adding oxygen or air to the gases to liberate the heat by internal combustion or by designing the catalyst vessel so that heat can be supplied to it from outside. In subsequent stages the carbon monoxide may be converted by reaction with steam to carbon dioxide and hydrogen and the carbon dioxide removed; these subsequent stages may be carried out by means that are well-known. This process has the feature that a low proportion of steam to hydrocarbon can be used, for example 1.6 or 2 lb. per lb. of distillate, which is near the theoretical minimum imposed by the need to avoid carbon deposition by the decomposition of the carbon monoxide formed.

The present invention is concerned with a further method of supplying the heat which is absorbed by the endothermic second-stage reactions, namely, as sensible Patented Apr. 29, 1969 "ice heat in the ingoing gaseous mixture, imparted by passing the mixture leaving the first gasification stage through at least one further stage which comprises an externallyfired preheater.

Accordingly the invention provides -a process for the production of gases containing methane by reaction of the vapour of parafiinic hydrocarbons having an average of from 4 to 15 carbon atoms per molecule with steam, which process comprises passing the hydrocarbon vapour with steam at a temperature of at least 350 C. into a bed of a nickel catalyst, whereby the maximum temperature in the catalyst bed is maintained at not more than 600 C. and substantially no carbon deposition takes place on the catalyst, passing the gaseous mixture produced in this gasification stage through at least one pair of stages being, first, an externally fired preheater, and, secondly, a reforming stage in which the preheated gases are subjected to the action of a reforming catalyst at such a temperature above 550 C. as to bring about the conversion of methane contained therein by reaction with steam into carbon monoxide and hydrogen whereby the methane content of the mixture is reduced to the desired value, the preheater providing heat which is absorbed by this endothermic reaction. It is preferred that the maximum temperature in the catalyst bed in the gasification stage shall be maintained at not more than 575 C. or 550 C.

Supplying the heat prior to the reforming stage or stages in this manner depends upon the gaseous mixture haying sufiicient thermal capacity to absorb the quantity of heat required by the reforming reactions without its temperature having to be raised to an impracticably high value. An impracticably high temperature is one for the attainment of which currently available heat-resisting steels would not have a commercially adequate safe working life (say 50,000 hours) at the desired operating pressure. For such steels, with operation at 25 atmospheres pressure, 800 C. can be regarded as the maximum temperature to which the gaseous mixture leaving the gasification stage can be preheated. It will be appreciated that higher temperatures Will be attainable as heat-resisting steels of improved properties become available.

When the proportion of steam to hydrocarbon is near the minimum, as described above, the thermal capacity of the mixture of hydrogen, carbon oxides, methane and undecomposed steam produced by the gasification stage is generally insufiicient for the method of the present invention within the limits described. When some increase in the proportion of steam in the mixture supplied to the reforming stage or stages, provided either by adding the additional steam to the mixture of gases and nudecomposed steam emerging from the gasification stage, or by increasing the proportion of steam to hydrocarbon in the mixture supplied to the gasification stage, for example, to at least 2.0 lb./lb., supplying the heat for the endothermic reforming reactions by preheat becomes practicable.

Increasing the proportion of steam mixed with the vapour of the hydrocarbons entering the gasification stage enables this mixture to be preheated to a higher temperature. With the higher proportion of steam, the reactions in this stage may become endothermic so that undesirably high catalyst temperatures (e.g. above 575 or 600 C.) can be avoided even with preheat temperatures up to 550 C. or 600 C. There is thus less risk of thermal decomposition leading to carbon deposition on the catalyst of even the heavier hydrocarbon molecules, as well as of decay of the catalyst due to the formation of a coating of hydrocarbon polymer on its surface.

In a preferred form of the invention, the heat which is supplied by preheating the mixture of gases and undecomposed steam is entirely sufiicient for the reactions in the reforming stage or stages. As compared with one alternative, of adding air, the present process avoids the necessity of consuming energy in compressing the air. Further, nitrogen is not a component of the final gas, which accordingly has a higher flame speed; and less carbon dioxide has to be removed in attaining the desired gas composition. As compared with another alternative, that of externally heating the catalyst enclosed in tubes, narrower tubes can be used when it is the gas mixture alone which is heated; the tubes can accordingly be thinner in the wall and so be subject to less thermal stress.

In carrying out the present invention, there may be one preheating and one reforming stage, or there may be two or a plurality of such pairs of stages.

One preferred embodiment of the invention is thus a process for the production of combustible gas containing methane, for example town gas, and, in particular, town gas of calorific value 500 B.t.u. per cu. ft., in which the vapour of light petroleum distillate or similar mixture of hydrocarbons containing an average of 4 to or carbon atoms per molecule substantially free from sulphur compounds is mixed with steam and the mixture is preheated, for example, to 550 C. or 600 C. and admitted to a first, gasification, vessel containing a catalyst whereby a gas rich in methane is produced, and in which the methane-rich gas is heated in an externally fired preheater to a temperature at which its sensible heat is entirely sufiicient to enable it to be reformed in a second, reforming, vessel containing catalyst to a gas of composition such that after carbon monoxide conversion by known means and partial carbon dioxide removal it has the desired calorific value and combustion characteristics.

This embodiment is illustrated by the following example (which has been calculated):

Outlet Stage 1 Outlet Stage 2 C O; 9. 01 10. 13 C 0.. O. 34 1. 68 10. 18 20. 31

Carbon monoxide conversion and partial carbon dioxide removal results in the production of gas of calorific value 500 B.t.u. per cu. ft. and Wobbe Number 715.

In a second embodiment of the invention, which enables substantially lower proportions of steam to hydrocarbon to be used, the methane-rich gas leaving the gasification stage is preheated in a first preheater, passed over a bed of catalyst in a first reforming stage, preheated again in a second preheater and passed over another bed of catalyst in a second reforming stage. These pairs of operations need not be limited in number although normally two will sufiice. If desired, and as is the case in the following illustrative example, which has been calculated, the plurality of preheating stages may be heated in the same furnace, with preheat at each step to the same temperature, but this is not necessary. The procedure enables more heat to be supplied for reforming within the same limits of temperature than when there is only one preheating operation.

4 EXAMPLE 2 Pressure of operation atm. 25 Light petroleum distillate, F.B.P. C Steam/distillate ratio lb./lb-.. 2.5 Stage 1 (gasification):

Preheat temperature C 530 Outlet temperature C 531.8 Preheat temperature for both reforming stages Outlet temperatures of reforming stages:

No. 1 C 626.8

No. 2 C 662.0

Gas compositions, percent, by volume:

Outlet Stage 1 Outlet reformer stages (gasifieation) N o. 1 No. 2

Carbon monoxide conversion and partial carbon dioxide removal results in the production of gas of calorific value 500 B.t.u. per cu. ft. and Wobbe Number 715.

In both the following examples, the hydocarbon feedstock was substantially completely freed from sulphur compounds after evaporation and before admixture with steam for the gasification stage of the process. This is a normal step where nickel catalysts are used. The tests described are much shorter than was possible, for steady conditions were established which would have persisted for as long as the nickel catalyst of the gasification stage remained active.

The catalyst used in the gasification stage in both examples was the co-precipitated alkali-containing nickelalumina catalyst E described in our application No. 351,- 190. The reforming catalyst used in all the reforming stages was a commercially available nickel catalyst made by the firm of Girdler and known as G. 56.

In both examples, the sensible heat imparted to the gases entering the reforming stages was entirely sufficient to support the endothermic reactions.

EXAMPLE 3 The experiment illustrates the embodiment of the invention in which only one reforming stage is used. Operating conditions were maintained steady for 470 hours before the test was terminated and the results were obtained during a sampling period which began about 250 hours after the start.

Reforming stage, catalyst column:

Diameter in 6 Height in 132 Preheat temperature C-.. 751 Outlet temperature C 621 Composition of outlet gas, percent by volume:

CO 8.9 CO 1.4 H 19.35 CH, 17.75 H O 52.6

Town gas can be produced from the gas leaving the reforming stage by carbon monoxide conversion, partial carbon dioxide removal, and condensation of undecomposed steam.

EXAMPLE 4 The experiment illustrates the embodiment of the invention in which there are two reforming stages with preheat to the mixture of gases and steam before each stage. Operating conditions were maintained steady for 468 hours before they were deliberately altered and the results were obtained during a sampling period which began 336 hours after the start.

Pressure p.s.i.g 350 Light distillate:

Specific gravity 0.721 Final boiling point C 178 Stage 1 (gasification), catalyst column:

Diameter in 5% Height in 46 /2 Rates of supply:

Light distillate lb. per hr 165.6 Steam lb. per hr-.. 401.0 Steam/ distillate ratio 2.42 Preheat temperature C 525 Outlet temperature C.. 540 40 Composition of outlet gas, percent by volume:

C0 1 1.1 CO 0.5 z 12.25 CH; 26.45 H O 49.7

Reforming stages No. 1 No. 2

Catalyst column:

Diameter (in.) 6 6 Height (in.) 72 72 Preheat temperature, 760 770 Outlet temperature, C- 620 668 Composition of outlet gas, percent by volume:

a mixture of steam and a hydrocarbon vapour in the ratio of at least 2.021 lbs. and at a temperature of at least 350 C. into a bed of a nickel catalyst, the maximum temperature in the catalyst bed being maintained at not more than 600 C. whereby substantially no carbon deposition takes place on the catalyst, passing the gaseous mixture produced in this gasification stage through at least one pair of stages comprising first, an externally fired preheater, and secondly, a reforming stage in which the preheated gases are subjected to the action of a reforming catalyst at a temperature above 550 C. to bring about the conversion of methane contained therein by reaction with steam to form carbon monoxide and hydrogen whereby the methane content of the mixture is reduced, the preheater providing heat which is absorbed by this endothermic reaction.

2. A process as claimed in claim 1 wherein further steam is added to the mixture produced in the gasification stage before it is preheated.

3. A process as claimed in claim 1 wherein the further heat which is supplied by preheating the mixture of gases and undecomposed steam is entirely sufiicient for the reactions of the reforming stage.

4. A process as claimed in claim 1 wherein there is one preheating and one reforming stage following the gasification stage.

5. A process as claimed in claim 1 wherein there are two pairs of preheating and reforming stages following the gasification stage.

6. A process as claimed in claim 1 wherein the maximum temperature in the catalyst bed in the gasification stage is maintained at not more than 575 C.

7. A process as claimed in claim 6 wherein the maximum temperature is 550 C.

8. A process for the production of town gas containing methane by reaction of the vapor of paraffinic hydrocarbons having an average of from 4 to 15 carbon atoms per molecule with steam, which process comprises passing a mixture of steam and a hydrocarbon vapor in the ratio of 2.42 to 3.5 :1 lbs. and at a temperature between 525 and 540 C. into a bed of a nickel catalyst, the maximum temperature in the catalyst bed being maintained at not more than 600 C. whereby substantially no carbon deposition takes place on the catalyst, passing the gaseous mixture produced in this gasification stage through at least one pair of stages comprising first, an externally fired preheater, and secondly, a reforming stage in which the preheated gases are subjected to the action of a reforming catalyst at a temperature between 621 and 798.5 C. to bring about the conversion of methane contained therein by reaction with steam to form carbon monoxide and hydrogen whereby the methane content of the mixture is reduced, the preheater providing heat which is absorbed by this endothermic reaction.

References Cited FOREIGN PATENTS 772,788 4/ 1957 Great Britain. 820,257 9/1959 Great Britain. 993,974 6/ 1965 Great Britain.

MORRIS O. WOLK, Primary Examiner.

R. E. SERWIN, Assistant Examiner.

US. Cl. X.R. 48196 

